OBJECTIVES: 1. The size of the DNA in the photoreceptor cells of the rabbit retina will be conducted as the post-irradiation time increases for rabbits that were exposed to acute doses of 60Co gamma-rays at the age of 6 weeks. These experiments are part of a long-term study that is nearing completion. 2. The relationship between the in vitro lifespans of primary cultures of rabbit fibroblasts and the ages of the donors is being explored both cross-sectionally and longitudinally with animals aged throughout the lifespan of the rabbit in captivity (5-7 yrs). These experiments are also part of a long-term study that is nearing completion. 3. In the irradiated animals, concomitant changes in tissues proximate to the retina, viz. lens, cornea, hair follicles, frontal lobes of brain, are also being investigated so that the life shortening in these tissues can be ascertained. Such studies are mutually complementary with experiments of the effects of heavy ions that are being conducted for NASA. 4. Preliminary investigations into possible changes in the size of the DNA in the rabbit cerebellum with age will also be implemented.